1. Field of the Invention
The present invention relates to a sample holder for a glow discharge mass spectrometer. More particularly, it relates to a sample holder which holds a sample to be analyzed by glow discharge mass spectroscopy for analyzing trace element(s) contained in a highly pure sample such as a metal, semiconductor or ceramic sample, and, further, the sample holder electrically insulates the sample from an anode.
2. Description of the Related Art
In a glow discharge mass spectrometer, an insulating sample holder, which is preferably in the form of a cone, is made of an insulating material for electrically insulating a sample which acts as a cathode from an anode. As the insulating material, polytetrafluoroethylene (hereinafter referred to as "PTFE") is preferably used, since it is easy to process and has good insulating properties and chemical resistance to a chemical used for cleaning a surface of the holder such as an acid.
FIG. 1 shows a cross sectional view of a typical glow discharge source, which comprises an insulating sample holder 1, a sample 2, an anode 3, a metal chuck 4, an ion exit slit 5 and a gas inlet 6. Glow discharge is generated in a gap between the sample, which is held by the metal chuck, 4 and the anode 3, where the sample acts as the cathode. The insulating sample holder electrically insulates the anode from the metal chuck 4 and the sample 2. Ions generated by glow discharge exhaust from the ion exit slit 5 into a mass spectrometer (not shown).
As described above, the sample holder 1 is conventionally made of PTFE.
When the glow discharge source is used, the atmosphere is evacuated to a pressure of about 1 to 5.times.10.sup.-8 Torr before discharge. Thereafter, a very small amount of argon gas is supplied from the gas inlet 6 into the glow discharge source, and then the discharge is started. In FIG. 1, when the sample holder is made of PTFE, air or some other gas is trapped in the pores of the PTFE material, even after the glow discharge source is evacuated for a long time, since the PTFE material is very porous. Therefore, for a long time after the start of glow discharge, ion species of residual gases such as N.sup.+, O.sup.+ and CO.sup.+ are detected with high intensity. Since these ions may cause interferences in analysis, it is necessary to wait untill the intensities of the residual gases decrease before analysis of trace elements such as S, Si and Fe. This waiting time decreases the efficiency of the analysis. Since PTFE contains fluorine atoms, fluorine-containing ion species such as .sup.19 F.sup.+ and .sup.31 CF.sup.+ are generated and cause interferences in analysis like the above described ion species.
In addition, after repeated measurements, the tip end of the sample holder is severely damaged and becomes largely uneven. Therefore, deposited materials on the tip end are not removed by washing with an acid and remain on the surface of the sample holder. Further, whisker-like materials are formed on the surface of the sample holder which cause abnormal discharge during measurement.